Japanese Laid-Open Patent Publication Nos. 2002-227688 and 2003-20930 describes a technique that is used when particulate matter (PM) accumulates in a filter, which is arranged in an exhaust system of a diesel engine. The particulate matter is burned and eliminated from the filter.
More specifically, the air-fuel ratio of the exhaust is repeatedly adjusted between a rich state and a lean state so that the temperature of the exhaust purification apparatus becomes higher than during normal purification. In addition, when the air-fuel ratio of the exhaust is in a lean state, active oxygen is released from the catalyst of the exhaust purification apparatus. This burns up the particulate matter accumulated in the filter.
In this burn-up process, a large amount of fuel is released into the exhaust within a short period of time when the air-fuel ratio of the exhaust is in a rich state. Accordingly, the burn-up process must be prohibited when the engine is operating in a state in which the temperature of the exhaust is low. A map may be used to determine whether the temperature of the exhaust is low when the engine is operating normally. However, when the engine is in an operation transition state, there may be a case in which the exhaust temperature is determined as not being low in accordance with the map even if the actual temperature of the exhaust purification apparatus is low and the catalyst may be inactivated.
In such a case, when a large amount of fuel is released into the exhaust within a short period of time, fuel is not sufficiently burned (oxidized) in the exhaust purification apparatus. As a result, incomplete combustion of the fuel may produce a large amount of particulate matter that further accumulates in the filter.